PCB design is a critical part of the entire PCB manufacturing process. This article mainly focuses on PCB stray capacitance, factors affecting PCB stray capacitance, calculation of PCB stray capacitance, and methods to eliminate PCB stray capacitance.
PCB design is a critical part of the entire PCB manufacturing process. This article mainly focuses on PCB stray capacitance, factors affecting PCB stray capacitance, calculation of PCB stray capacitance, and methods to eliminate PCB stray capacitance.
1. What is Stray Capacitance?
As the name implies, "stray" means unregulated, random, and out of place. Therefore, stray capacitance can be defined as the unavoidable, unintended, and unwanted capacitance existing between various parts of a circuit.
Capacitance does not only exist inside capacitors. In fact, any two surfaces at different potentials that are close enough to generate an electric field exhibit capacitance, just like a physical capacitor.
This effect commonly exists in circuits. For example, the unintended capacitance between conductive traces and component leads is referred to as stray capacitance.
Stray capacitance may interrupt the normal current flow within a circuit.
2. Calculation of PCB Stray Capacitance
In general calculations, the formula for stray capacitance is C = Q/V, which measures the charge accumulated at a differential potential.
In PCB design, the formula for stray capacitance is modified to:
C = εA/D
This formula describes the relationship between the capacitance value, the dielectric constant of the insulator, the overlapping area, and the distance between conductors.
\(C=\frac{0.00885 \times E_r \times A}{d} \quad (\text{pF})\)
- A: Trace area (height × width), unit: mm²
- d: Clearance between traces, unit: mm
- \(E_r\): Relative dielectric constant (relative to air)
3. Factors Affecting the Magnitude of PCB Stray Capacitance
Three factors determine the amount of stray capacitance generated in a capacitive structure. All of these factors influence stray capacitance by affecting the electric field flux generated by a given electric field force (voltage between any two plates).
3.1 Plate Spacing
With all other factors constant, the larger the spacing between plates, the smaller the stray capacitance.
Conversely, smaller plate spacing generates more stray capacitance.
Tighter plate spacing leads to higher electric field force, which produces relatively high electric field flux (charge accumulation on both plates) for any given voltage applied to the two plates.
[Illustration Position: PCB Diagram]
3.2 Plate Area
With all other variables constant, a larger plate area results in more stray capacitance, while a smaller plate area generates less stray capacitance.
3.3 Dielectric Material
With all other variables constant, a dielectric material with a higher dielectric constant produces larger stray capacitance, and a dielectric material with a lower dielectric constant produces smaller stray capacitance.
The relative dielectric constant represents the dielectric constant of a material relative to a vacuum (pure vacuum). For example, glass with a relative dielectric constant of 7 has a standard dielectric constant 7 times that of a vacuum. Thus, with all other variables equal, it generates 7 times stronger electric field flux than a pure vacuum.
4. How to Reduce Stray Capacitance in PCB Design?
In many applications, stray capacitance between multiple signal traces may degrade or impair the performance of the entire design.
Stray capacitance is often negligible at low frequencies but becomes a major circuit issue at high frequencies. Stray capacitance can be minimized during PCB layout.
Stray capacitance is typically caused by electrical coupling between a signal trace and another signal trace, or between the substrate and a signal trace.
The following are methods to reduce stray capacitance:
4.1 Keep Component Leads Extremely Short
Keeping the leads of electronic components very short and grouping components in a way that eliminates capacitive coupling can reduce the generation of stray capacitance.
For a simple example: new inductors purchased from manufacturers usually have very long leads, which can extend several inches from the inductor body. However, inserting such inductors with long leads into a circuit may cause problems.
When the long leads of an inductor are placed close to each other, these leads essentially act as wires. When wires in a circuit are placed close to one another, a capacitive effect occurs, and even a small number of wires can generate considerable capacitance.
Stray capacitance can block low-frequency signals because capacitive components have high impedance to low-frequency signals, making it difficult for low-frequency signals to pass through circuits with capacitive characteristics. Adding unwanted capacitance to a circuit can block low-frequency signals, and in the case of wireless or audio circuits, it may block the entire frequency range of the circuit.
Therefore, inductor leads must be kept short (ideal length < 1.5 mm) to effectively suppress the capacitive effect caused by stray capacitance, which limits the inductor’s ability to pass low-frequency signals.
Surface-mount inductors are more suitable for circuits because their lead-free terminals are directly mounted on the power plane of the circuit, nearly eliminating all capacitance. Lead-free surface-mount devices avoid capacitive limitations and enable inductors to transmit low-frequency signals better in circuits.
4.2 Increase Spacing Between Components/Assemblies
Increasing the spacing between components, assemblies, traces, cables, and other electrical nets is critical to reducing the generation of stray capacitance.
Stray capacitance is inversely proportional to distance: a larger distance results in smaller stray capacitance, and a smaller distance results in larger stray capacitance.
4.3 Add Shielding Conductors
Place a reference signal (i.e., a shielding conductor) between various nets with low design requirements. For example, add a grounded copper strip between adjacent traces.
The copper strip provides shielding, prevents charge accumulation, and thus reduces the generation of stray capacitance.
4.4 Reduce Trace Width
Stray capacitance increases as the cross-sectional area of a conductor increases. Therefore, reduce the trace width, especially for traces carrying high-frequency signals.
4.5 Remove Inner Ground Planes
Large inner ground planes are suitable for thermal management and EMI control but provide no benefit for reducing stray capacitance.
Removing inner ground planes is recommended to mitigate stray capacitance.
4.6 Avoid Excessively Parallel Metal Routing
The above is a brief introduction to PCB stray capacitance and techniques for reducing PCB stray capacitance. We hope this article is helpful to you.
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